What are the stains? Washing up liquid gets paint out if you scrub it before washing in the machine. For tomato-based food stains I soak and scrub first before putting in the machine. Make sure you use warm water as hot can 'set' the stain. Also sunlight does wonders as Kate said.
Vanish power gel stick (it looks like a roll on deodorant), best thing that works!
I leave them in the sun (when we finally have some) while they are wet and they come out straight away!